This invention relates to the recovery of precious metals from carbonaceous and sulfidic ores or concentrates which are refractory to standard cyanidation techniques. In the context of this disclosure, "refractory ore" is one which will not readily allow precious metal extraction therefrom by direct cyanidation, nor, in some cases, by carbon-in-leach cyanidation (CIL) or carbon-in-pulp (CIP) cyanidation. The invention relates, more particularly, to an improved process for the treatment of these ores to obtain consistently high precious metal recoveries.
In the recovery of precious metals from mineral sources with which the precious metal is associated, a number of steps and combinations of steps have been proposed to improve the yield. As recoverability is a function of the refractoriness of the ore, any added step must be economically justifiable in the recovery of additional amounts of, e.g., gold.
An ore that is refractory due to its carbonaceous content can often be treated by chlorination prior to carbon-in-leach cyanidation. This has been done commercially for a number of years using 40 to 100 lbs. of chlorine gas per ton of ore. As the amount of carbonaceous material increases, so does the amount of chlorine gas consumed. A point is soon reached, however, wherein the process is not economically feasible.
An ore that is refractory due to its sulfide content can often be treated by various oxidation pre-treatments prior to cyanidation or carbon-in-leach cyanidation. Commercial pre-treatment schemes include roasting, autoclaving in the presence of oxygen-containing gas and bacterial leaching.
An ore that is refractory due to both its carbonaceous and sulfide content represents a more complex problem. Chlorination followed by carbon-in-leach cyanidation will extract most of the precious metals, e.g., gold, but requires an exorbitant amount of chlorine gas. For example, from 400 to 900 lbs. of chlorine gas per ton of ore may be required. Roasting followed by carbon-in-leach cyanidation typically will extract 80% to 85% of the gold in the ore, but this often requires roasting temperatures of 650.degree. C. Autoclaving in presence of oxygen followed by carbon-in-leach cyanidation will extract 70% to 80% of the gold in the ore, as will bacterial leaching.
The standard method in the industry for extracting precious metals such as gold from these ores is cyanidation. Cyanidation, carbon-in-leach cyanidation and carbon-in-pulp cyanidation have been coupled with various pre-treatment procedures in attempts to improve the results of the recovery of precious metals. A number of these pre-treatment steps have been carried out under atmospheric conditions or in an autoclave. Some of the shortcomings of these prior art pre-treatments have included the undue consumption of the materials with which the ore has been treated and other unacceptable consequences in subsequent treatment steps. Additional shortcomings have been an undue increase in the leaching time and temperature constraints which are unacceptable for the recovery of precious metals.
The potential or ultimate amount of precious metal in the ore which could be recovered is a goal against which all attempts have been measured. This ultimate goal has eluded many attempts, especially on an industrial scale, and has been an incentive for a number of investigations. Such potentially complete recovery, although often alleged, has been mere speculation or economic nonsense. Hence, with respect to the autoclave pre-treatment with oxygen coupled with carbon-in-leach or carbon-in-pulp cyanidation, various pre-treatments have fallen short of a complete exhaustion or substantially complete exhaustion of precious metal in the ore.